Structure, bonding, and anharmonic librational motion of CO on Ir{100}

A combined low energy electron diffraction (LEED)—density functional theory (DFT) study of the structure of the Ir{100}-c(2×2)-CO phase provides a comparison of the two techniques for a simple molecular adsorbate. Both studies clearly identify atop adsorption and agree on the key structural parameters: a strong buckling of the first Ir layer, a short Ir–C bond length, and a slight lengthening of the CO bond. The molecule is found to be adsorbed in an upright configuration, although an incomplete treatment of the correlated vibrational motion of the CO molecule across the surface in the LEED analysis results in an apparent tilt of 8° from the surface normal. The DFT study determines a high adsorption energy of 2.65 eV for the c(2×2) phase which can be associated with the relief of the high tensile stress of the metastable Ir{100}-(1×1) phase and can be correlated with the short Ir–C bond. The 0.25 ML p(2×2)-CO phase displays an almost identical local bonding geometry but has a slightly lower adsorption ene...